This invention relates to the surface treatment of an article by the tumble process, and, more particularly, to a tumbling method and tumble medium having a radiographic signature different from that of the article.
The tumble process has been used for many years during the manufacture of a wide variety of articles for surface preparation or treatment. For example, tumbling has been used for abrading, polishing, rough cutting, deburring, edge radiusing, descaling, surface texture or property improvement, cleaning, and destressing, among others. Various types of tumbling systems used include barrel, vibratory, and centrifugal, alone or in combinations, with or without liquid. Certain components for gas turbine engines, for example blades, vanes and nozzles, are complex in shape and have precision requirements for surface finish, including edges. Therefore, it has been a practice to use the tumble process for surface treatment or preparation. Traditionally, such a process has been used primarily to remove burrs and for the rounding of sharp edges produced during manufacture, as well as to achieve required surface finish.
A detailed description of the tumble process is included in Manufacturing Enineers Handbook, Third Edition, published by McGraw - Hill Book Co. for the Society of Manufacturing Engineers. For example, details of processing, equipment and tumble media are included in Chapter 24 "Surface Preparation", section on Barrel and Vibratory Finishing found on pages 24 -13 through 24 -26. Typical U.S. Patents describing some facets of the tumbling process and various forms and shapes of tumble media pellets include 2,947,124 --Madigan et al. (Aug. 2, 1960); 3,239,970 --Bishop (Mar. 15, 1966); 4,712,333 --Lofton (Dec. 15, 1987); and 5,090,870 --Gilliam (Feb. 25, 1992); among others.
Modern gas turbine engine components, such as blades, vanes and nozzles operating in the higher temperature sections of the engine, for example the turbine section, are manufactured to include hollow interiors for air cooling. Internal air cooling passages frequently are labyrinthine in form and are connected through surface openings to the exterior of the component for the discharge of cooling air. Typical examples of such components are shown in U.S. Pat. Nos. 5,387,085 --Thomas, Jr. et al. (Feb. 7, 1995); 5,458,461 --Lee et al. (Oct. 17, 1995); and 5,503,529 --Anselmi et al. (Apr. 2, 1996). In a typical tumble process, such components or parts are finished by immersing the parts in a container of abrasive media or stones, generally of a ceramic material. When a motion or vibration is imparted to the container, the media moves against the surface of the part, in this example made of a metal or metal alloy, and deburrs, finishes or otherwise treats the surface of the part. During the course of such a tumbling process, for example with an air cooled blade, root plugs in the blade dovetail holes through which cooling air is introduced into the blade can loosen and fall out during tumbling. Some of the media have been observed to wear or break into smaller pieces which have entered the internal cavities of the part, for example through such cooling air openings in the surface.
Conventional tumble media are made primarily of alumina and/or various silicates which are not detectable from the material of the part by radiography, one example of which is the x-ray technique. Another example is the detection of rays from radioactive substances. Tumble media remaining in the internal passages of such high temperature operating parts restrict the flow of cooling air, creating a "hot spot" that can lead to premature part failure.